Many pulmonary toxins exert discrete and localized reactions within the lungs suggesting that the heterogeneous population of lung cells vary considerably with respect to interactions with xenobiotics. In order to elucidate the differential effects of various toxins on the lung cell populations, techniques for isolating and characterizing the major cell-types from a number of animal species are being developed. The cell-types of principal interest include the vascular endothelial cells, alveolar type I and type II cells, and the inter-stitial fibroblasts and pericytes which constitute over 90% of the lung mass. Further, the bronchiolar Claral cells and alveolar macrophages are also of interest because of their known metabolic activities. Cells are characterized after dispersing the lung tissue into a single cell suspension with mild collagenase treatment. They are separated principally by centrifical elutriation using microprocessor control flow and/or density gradients. This technique permits the separation of heterogeneous cell populations not only according to size but also according to density. Cell-types are identified using specific histochemical and biochemical markers. Parallel studies into the characteristic morphology of the major cell-types are under way utilizing electron microscopy. From rabbit lung, both alveolar type II and bronchiolar Clara cells have been isolated. These cell-types are presently being used to quantify the distribution of the pulmonary cytochrome P-450 system using acetylaminofluorene as substrate. Considerable work into isolating rat lung endothelial cells has established a number of enzymatic markers that are being used to isolater and purify this cell-type. The endothelial cells will be used to study the effects of various toxins on the integrity and function of these cells.